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SAE-AISI P21 (T51621) Low-Carbon Mould Steel

SAE-AISI P21 steel is a tool steel. Cited properties are appropriate for the hardened (H) condition. P21 is the designation in both the SAE and AISI systems for this material. T51621 is the UNS number. The graph bars on the material properties cards below compare SAE-AISI P21 steel to: tool steels (top), all iron alloys (middle), and the entire database (bottom). A full bar means this is the highest value in the relevant set. A half-full bar means it's 50% of the highest, and so on.

Mechanical Properties

Elastic (Young's, Tensile) Modulus

190 GPa 27 x 106 psi

Poisson's Ratio


Rockwell C Hardness


Shear Modulus

73 GPa 11 x 106 psi

Tensile Strength: Ultimate (UTS)

1130 MPa 160 x 103 psi

Thermal Properties

Latent Heat of Fusion

250 J/g

Melting Completion (Liquidus)

1450 °C 2650 °F

Melting Onset (Solidus)

1410 °C 2580 °F

Specific Heat Capacity

470 J/kg-K 0.11 BTU/lb-°F

Thermal Conductivity

41 W/m-K 24 BTU/h-ft-°F

Thermal Expansion

12 µm/m-K

Electrical Properties

Electrical Conductivity: Equal Volume

7.9 % IACS

Electrical Conductivity: Equal Weight (Specific)

9.0 % IACS

Otherwise Unclassified Properties

Base Metal Price

6.5 % relative


7.9 g/cm3 490 lb/ft3

Embodied Carbon

2.3 kg CO2/kg material

Embodied Energy

32 MJ/kg 14 x 103 BTU/lb

Embodied Water

62 L/kg 7.4 gal/lb

Common Calculations

Stiffness to Weight: Axial

13 points

Stiffness to Weight: Bending

24 points

Strength to Weight: Axial

40 points

Strength to Weight: Bending

31 points

Thermal Diffusivity

11 mm2/s

Thermal Shock Resistance

37 points

Alloy Composition

Iron (Fe) 92.1 to 94.2
Nickel (Ni) 3.9 to 4.3
Cobalt (Co) 1.1 to 1.3
Manganese (Mn) 0.35 to 0.72
Silicon (Si) 0.2 to 0.4
Chromium (Cr) 0 to 0.5
Carbon (C) 0.18 to 0.22
Vanadium (V) 0.15 to 0.25
Copper (Cu) 0 to 0.25
Phosphorus (P) 0 to 0.030
Sulfur (S) 0 to 0.030

All values are % weight. Ranges represent what is permitted under applicable standards.

Followup Questions

Similar Alloys

Further Reading

ASTM A681: Standard Specification for Tool Steels Alloy

Tool Steels, 5th ed., George Roberts et al., 1998

Tool Steels: Properties and Performance, Rafael A. Mesquita (editor), 2016

ASM Specialty Handbook: Carbon and Alloy Steels, J. R. Davis (editor), 1996